Phenolic foam is useful in various construction materials because of its superiority among resin foams particularly in flame retardance, heat resistance, low fuming properties, dimensional stability, solvent resistance, and fabricability. Phenolic foams are widely used in building applications in view of their thermal insulation and fire resistant properties.
Phenolic foams are thermally stable over a broad temperature range, maintaining performance and stability from −196° C. up to 200° C. The thermal conductivity of phenolic foam is low, which has led to a broad range of applications as an insulating material. Finally, phenolic foam is highly resistant to chemicals and solvents.
Phenolic foam, however, is extremely friable (A. H. Landrock (ed.), Handbook of Plastic Foams, Noyes Publications, New Jersey, 1995). As a consequence, phenolic foam has rarely been used as a core material in sandwich structures because the skin will not stay bonded. The extreme friability stems from the inherently brittle nature of the phenolic component. Phenolic foam of relatively low density (lower than 200 kg/m3, suitable for consideration for weight-critical aerospace applications) crumbles readily, causing serious problems when used in structural sandwich panels. Frequently reported failures include skin debonding and susceptibility to damage during handling, causing excessive dust in the workplace. Friability is an important property of low-density foams, and is measured by mass loss due to surface abrasion and impact damage. For phenolic foam with a density below 100 kg/m3, the friability is so high that severe problems arise in production and applications. For example, the friability of phenolic foam reportedly causes dust pollution in production areas and difficulties in bonding to other materials. Vibrations in service applications also cause problems that restrict and often preclude its use in structural applications with even the most modest load-bearing requirements. The friable nature of phenolic foam is believed to stem from the brittleness of the material (Landrock (ed.), Handbook of Plastic Foams, Noyes Publications, Park Ridge, N.J., USA, 1995; and Mao et al., Chem. Ind. Eng., 15(3):38, 1998). Consequently, the poor bond strength of phenolic foam when bonded to other materials has severely restricted its use in structural applications.
Phenolic foam receives much attention in fields where fire resistance is critical, such as building materials for civil construction, passenger and military aircraft, and naval vessels. However, structural applications of phenolic foam have been severely limited because of the extreme brittleness and friability. Since the late 1970s, some effort has been devoted to increasing the toughness of phenolic foam, as reviewed in Mao et al. (Chem. Ind. Eng., 15(3):38-43, 1998) and Knop an Scheip (Chemistry and Application of Phenolic Resins, Springer-Verlag, New York, 1979). Primary efforts have been devoted to identifying and incorporating chemical modifiers that impart flexibility and toughness to phenolic foam. Unfortunately, these efforts either were of little effect or severely compromised the desirable flammability, smoke density and toxicity (FST) properties. A second approach to increase toughness of phenolic foam has involved the addition of certain inert fillers. Finely ground fillers such as carbon black, talc, mica, asbestos, wood and cork flours typically improved the texture and homogeneity of foams, but these fillers generally led to much higher density foams (U.S. Pat. No. 2,446,429).
Currently, polyvinyl chloride (PVC) and polyurethane foams are popular choices for sandwich cores in structural applications. PVC foam is stiff and strong relative to most other foams, while polyurethane foam possesses medium stiffness and ease of processing. Both foams are widely used as sandwich core materials. However, polyurethane foam is flammable and produces toxic fumes during combustion. Though PVC foam has relatively low flammability, it releases toxic halogen-bearing gas under fire conditions. As material FST (flammability, smoke density and toxicity) standards become increasingly stringent worldwide, limitations of conventional structural foams may preclude their continued use.